1. FIELD OF THE INVENTION
The present invention relates to stress-sensitive semiconductor devices of the type which have become known in the art as semiconductor strain gauge elements, strain gauges, or strain sensors. More specifically, the invention relates to the so-called silicon strain sensors, wherein the strain is sensed by a pattern of one or more piezoresistive elements or resistors which are formed in a body or a layer of silicon. Such a resistor pattern produces an electrical resistance change output signal which represents the strain experienced by the sensor. Still more specifically, the invention relates to those forms of such strain sensors which are particularly useful as the sensor or transducer elements of fluid pressure to electric signal transducers or transmitters.
2. DESCRIPTION OF THE PRIOR ART
Many forms and configurations of silicon strain sensors, and numerous forms of fluid pressure transducers employing such sensors, are known in the art. The usual practice in constructing such sensors is to form the desired piezoresistive resistors, by the usual diffusion or ion implantation techniques, in or on the surface or surfaces of a body of silicon having the dimensions that are required for the particular application. Such sensors will be referred to hereinafter as silicon on silicon sensors. The resistors so formed are generally connected as the arms of a bridge circuit. This may be done either on the sensor itself or external to the sensor. The bridge output signal represents the resistance changes or output signals of the resistors and hence the strain experienced by the resistors and the body, and is usually used to actuate appropriate indicating and/or controlling apparatus.
To permit the operation of the resistors in a bridge configuration, the resistors must be crystallographically oriented on the sensor body in such a pattern or patterns that a change in strain causes the resistances of certain of the resistors to change in the direction which is opposite to that in which the resistances of others of the resistors are changed. In the beam type sensor, this is usually accomplished by either orienting the resistors in both longitudinal and transverse crystallographic directions on the same surface of the body, or by orienting the resistors in the same crystallographic direction on oppositely strained surfaces of the body. Such crystallographic directions will be referred to hereinafter simply as directions.
An example of a typical one of such sensors employed in a typical pressure transducer is the beam type sensor and transducer combination which is disclosed in the Whitehead et al U.S. Pat. No. 3,780,588. An example of a typical diaphragm form of such sensors, for use in the diaphragm type of pressure transducer, is the sensor which is disclosed in the Frantzis U.S. Pat. No. 3,230,763.
It has been found that the operation of such silicon on silicon sensors becomes unreliable when the sensors are operated in environments which expose them to operating temperatures in excess of 230.degree. F. and/or to nuclear radiation. This is particularly true for those applications in which a high degree of measurement precision is required. It is known that this unreliability results mainly from the unavoidable presence of np or pn junctions which exist between the resistor pattern and the bulk silicon which carries it. Those junctions permit the flow of leakage currents which degrade the sensor output signals to an extent which is dependent upon the ambient temperature and radiation levels.
It has been proposed in the past to avoid the above-noted undesirable operation by replacing the silicon member or body with a corresponding body or substrate of sapphire, by growing an epitaxial layer of silicon on each surface of the sapphire member which is to carry a piezoresistive resistor pattern, and by forming each such a pattern from the corresponding silicon layer. Since the sapphire substrate in such a sensor construction is an electrical insulator, the above-noted np and pn junctions are not present. Accordingly, such a silicon on sapphire sensor construction is not subject to the subject to the above-noted leakage currents and unreliable operation.
Such a silicon on sapphire strain sensor has the practical disadvantage, however, of having only about one-half the sensitivity of its silicon on silicon counterpart. That is, a silicon on sapphire sensor possesses the shortcoming of producing only about one-half as much change in output effect or signal for a given change in applied force and deflection as is produced by an equivalent silicon on silicon sensor operating under the same conditions.